The present invention relates to a process for purifying a liquid hydrocarbon product by removing high molecular weight contaminants.
Many liquid hydrocarbon products are available, which contain high molecular weight contaminants. The contamination normally originates in the process producing the product and is due to undesirable polymerisation of substances intrinsically present in the process. Such polymerisable substances may be the liquid hydrocarbon product itself (e.g. styrene, butadiene, isoprene), but can also be one or more other monomers intrinsic to the process for producing the hydrocarbon product. Such other monomer may itself be a separate product of the process. The polymerisable substances have in common that they are prone to polymerisation under the process conditions. For instance, dicyclopentadiene and cis- and trans-1,3-pentadiene (piperylene) may contain small amounts of polyisoprene, because both are typically produced from the C5-cut of a cracked effluent stream from which isoprene is produced. Thus, isoprene is intrinsically present in the process producing piperylene or dicyclopentadiene and may form polyisoprene.
Furthermore, if a polymerisable hydrocarbon product (e.g. styrene) is used as a feedstock for the production of a homopolymer, copolymer or block copolymer (e.g. syndiotactic polystyrene, random styrene-butadiene copolymer or styrene-butadiene block copolymer), the product itself may be forming an uncontrolled polymeric contaminant under the right conditions. Apart from the fact that such polymeric contaminant may interfere in an undesirable manner with the formation of the envisaged polymer, it will also normally have a reactivity which is different from the envisaged polymer. This latter aspect could cause difficulties when processing the envisaged polymer into the endproduct.
The undesirable polymerisation may occur due to reactions caused by the presence of traces of substances forming polymerisation initiators which are difficult to remove or avoid. Such initiators could be radicals formed from peroxides, which in turn can be formed from oxygen ingress in the presence of olefins or other compounds having an unsaturated double bond (Cxe2x95x90C). Polymerisation may also occur due to the presence of substances which act as catalyst for the polymerisation reaction, especially where this catalyst is a substance intrinsic to the process.
The high molecular weight contaminants are typically present in relatively small amounts up to 5% by weight based on total weight of the liquid product. However, often amounts as low as 3% by weight or less or even 1% by weight or less already make the product less suitable or even completely unsuitable for subsequent processing or for direct end use.
In order to benefit from the process of the present invention, the liquid hydrocarbon products to be subjected to the process of the present invention will usually contain at least 0.00001% wt of high molecular weight contaminants, more specifically at least 0.001% wt. However, in some cases even smaller amounts can make use of the process of the present invention attractive.
In general, several separation techniques are known in the art for separating contaminants from the actual product based on the difference in molecular weight. One such known and applied technology is distillation, wherein a separation takes place on the basis of differences in boiling points between the various components, which differences are related to the differences in volatility between the various components. However, the fact that high molecular weight contaminants are present in such small amounts makes distillation a relatively expensive way of purifying the product. Namely, in distillation the actual product forming the bulk of the stream to be treated would have to be evaporated, recovered as the gaseous top fraction and subsequently condensed for further use, whereas the high molecular weight contaminants would have to be recovered as the liquid bottom fraction. Beside the energy-intensive nature of this treatment, a lot of equipment would also be required to carry out the distillation. Moreover, the high temperatures normally applied in distillation increase the likelihood of (thermal) decomposition of the hydrocarbon product. This could lead to product loss and fouling of the equipment. The application of high temperatures could be avoided by performing the distillation at low pressure, but this would increase the risk on oxygen ingress, which in return could lead to the formation of peroxides. As explained above, these peroxides will induce polymerization reactions. Furthermore, low pressure distillation would necessitate the use of more expensive equipment. It will be appreciated that these factors render the distillation unattractive from an economic perspective.
The present invention aims to provide a method for effectively and cost-efficiently removing the high molecular weight contaminants.
Accordingly, the present invention relates to a process for purifying a liquid hydrocarbon product comprising 5% by weight or less of high molecular weight contaminants having a molecular weight of at least 1000, said process comprising contacting the product stream with a hydrophobic non-porous or nano-filtration membrane to produce a purified product stream and recovering the purified product stream as permeate.